A transistor formed over a glass substrate or the like is manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in a liquid crystal display device. Although a transistor including amorphous silicon has low field-effect mobility, it can be formed over a larger glass substrate. On the other hand, although a transistor including polycrystalline silicon has high field-effect mobility, it is not suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel of a display device.
Patent Document 3 discloses a technique in which in a staggered transistor including an oxide semiconductor, a highly conductive oxide semiconductor including nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby the contact resistance between the oxide semiconductor and the source electrode and between the oxide semiconductor and the drain electrode is reduced.
Further, as a method for forming a source region and a drain region of a transistor including an oxide semiconductor by a self-aligned process, Non-Patent Document 1 discloses a method in which a surface of an oxide semiconductor is exposed and argon plasma treatment is performed thereon for reducing resistivity of the exposed portion in the oxide semiconductor.